A Kurdish man waves a large flag of the Kurdish People's Protection Units (YPG) political wing, the Democratic Union Party (PYD), during a demonstration against the exclusion of Syrian-Kurds from the Geneva talks in the northeastern Syrian city of Qamishli on February 4, 2016. (DELIL SOULEIMAN/AFP/Getty Images)

When it comes to many of the interlocking conflicts of the Middle East, the Kurds seem to be a kind of glue.

For decades, millions of Kurds have lived as part of stateless ethnic minorities across a vast expanse of the region, from Iran in the east to Turkey in the west. To varying degrees, they were marginalized, oppressed and under-represented.

Now, they're at the heart of the Middle East's most pressing crises. An autonomous Kurdish region in Iraq has proven to be perhaps the most stable corner of an otherwise imploding nation. Kurdish militias in Syria have battled both the advances of the Islamic State as well as other rebel factions. And a Kurdish separatist insurgency in Turkey has posed the U.S.'s NATO ally a huge strategic conundrum, and sowed deeper unrest in the country.

The aftermath of a Kurdish militant bombing in the Turkish capital Ankara last week, which led to the deaths of 28 Turkish military personnel, illustrated the headaches facing the West and especially Turkey, as my colleague Liz Sly reported over the weekend.

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In the U.S., the call to "arm the Kurds" has become a standard refrain of the presidential election campaign, with little recognition of the dizzying array of Kurdish factions operating in the midst of the region's crises. Not all are working in tandem -- some are in direct opposition to the other. What follows is a rough digest of the alphabet soup of prominent Kurdish groups in the Middle East.

TURKEY

The Kurdistan Workers’ Party, known by the abbreviation PKK, is the oldest and most entrenched Kurdish separatist group in Turkey. It emerged as a Marxist-Leninist organization, championing the cause for a Kurdish homeland, and waged a violent insurgency against the Turkish state. It’s considered a terrorist group by the U.S., E.U. and Turkey.

Since the early 1980s, the violence has claimed some 40,000 lives. The PKK focused most, but not all, of its attacks on security forces – methods that resurfaced since the resumption of hostilities last year after the collapse of a slow-moving peace process between Ankara and the militant group. Jailed leader Abdullah Ocalan remains a powerful influence, and his image still adorns graffiti and posters in the cities of Turkey’s restive southeast.

In recent months, the Patriotic Revolutionary Youth Movement, a paramilitary organization of young PKK sympathizers, and the Civil Protection Units have been involved in pitched street warfare in a number of towns currently under curfew. It’s unclear the extent to which they are acting in coordination with the PKK’s senior leadership, which operates euphemistically “in the mountains,” including camps in the rugged borderlands of northern Iraq.

Last week’s bombing in Ankara was claimed by the Kurdistan Freedom Hawks, a militant group known by the abbreviation Tak that was an offshoot of the PKK, but says its ties are now severed. It took responsibility for a spate of recent assaults, including a mortar attack on an Istanbul airport in December and a 2012 bombing of a bus carrying Turkish military personnel.

Meanwhile, Kurdish political aspirations are most prominently championed by the Peoples’ Democratic Party, known as the HDP. The political party – a motley coalition of leftists, Kurds, and other Turkish minorities – came to the fore in national elections last June, winning enough votes to break the parliamentary majority of the ruling center-right Justice and Development Party, or AKP.

The party was cheered on by onlookers abroad and at home, who saw its electoral gains as a sign of both the maturation of Kurdish nationalist politics and Turkish democracy as a whole.

The optimism of that moment faded quickly. The PKK’s hostilities with the Turkish state pushed the HDP, some of whose members have ties to the PKK, into a tricky spot.

It clings to a narrative of inclusion and peace, but has been demonized as a terrorist proxy by the ruling party and other nationalists. The initial success of the party, and its charismatic, youthful leader Selahattin Demirtas, clearly also irked elements within the PKK who were opposed to their compatriots’ entrance into the Turkish political mainstream.

Turkey’s significant Kurdish population is hardly monolithic. A considerable proportion of conservative, religious Kurds vote for the AKP, a party which has done more to boost Kurdish rights while in power than any other previous Turkish government.

A Kurdish Islamist group known as Hezbollah – no relation to Iran’s Lebanese proxy – has clashed violently in the past with supporters of the PKK and HDP. Among Kurds sympathetic to the PKK, Hezbollah’s fighters are referred to pejoratively as “contras,” foot soldiers for the state's counter-insurgency.

SYRIA

The most prominent Syrian Kurdish political organization is the Democratic Union Party, or PYD. The PYD has presided over the dramatic inroads made by Kurdish militias since the advent of Syria’s civil war, carving out a chunk of territory in northern Syria – an enclave hailed by Kurdish nationalists as Rojava. A separate political organization, the Kurdish National Council, receives backing from Iraqi Kurdistan and has vied for legitimacy with the PYD.

Turkey considers the PYD to be the Syrian branch of the PKK; the two groups do share a complex, intertwined history. A de facto Kurdish state just south of the border is a non-starter for Ankara. "We are only against the PYD establishing a corridor in northern Syria," declared Turkish President Recep Tayyip Erdogan earlier this month.

But neither the U.S. nor Russia – geopolitical adversaries in the region – fully share the same analysis. Over the past year, both governments have toyed with deepening support to the Syrian Kurds, who have proved among the most effective units combatting the ravages of the Islamic State.

The People's Protection Units, or YPG, are the militias associated with the PYD. A female arm is known as the YPJ. The YPG are currently in the midst of their own tactical offensive, seizing key bases from other rebel factions that led to a barrage of artillery fire from the Turkish military.

Last week, Turkey accused the YPG of carrying out the Ankara bus bombing, though it appears that the Syrian Kurd who detonated the explosives was in the employ of a different, Turkish-based organization, as explained above.

The militias have drawn in hundreds of Kurdish youth from towns in Turkey. Last year, WorldViews visited a cemetery in the city of Diyarbakir, the most important Kurdish-majority urban center in Turkey’s southeast, devoted to YPG fighters killed across the border in Syria.

Despite valiantly challenging the jihadists of the Islamic State, the YPG has been accused by rights groups of committing its own war crimes, including waging a campaign of demolition and displacement in areas seized by its units that's supposedly tantamount to ethnic cleansing.

IRAQ

The Kurdish Regional Government, headquartered in the northern Iraqi city of Erbil, administers a largely autonomous state within the borders of Iraq. Its fighting units, known as the peshmerga, have played a vital role as a bulwark against the Islamic State, which surged into parts of Iraq further south in 2014.

But the KRG itself is rife with factionalism between rival camps, as my colleague Loveday Morris reported last year. Two powerful, long-standing rival parties -- the Kurdish Democratic Party (KDP) and the Patriotic Union of Kurdistan (PUK) -- function as a ruling coalition yet command the fealty of different units of fighters.

Recently, the political preeminence of these two parties has been challenged by the Movement for Change, or Gorran, which seeks to stamp out the supposed corruption of the Iraqi Kurdistan's venerable status quo.

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